1. Field of the Invention
This invention relates to diode lasers and methods of fabrication of diode lasers.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The present invention pertains to diode lasers that operate at wavelengths between 470 and 630 nm—a spectral range poorly served by any diode laser technology. Such lasers would find wide use in scientific, biomedical, sensing, illumination and display applications, supplanting older technologies in existing markets and enabling new markets dependent on the unique advantages of diode lasers. These advantages include small size, low cost, high efficiency, and the capability for high-speed modulation.
Of particular interest is the green spectral range, approximately 495-570 nanometers (nm). When combined with commercially available blue and red diode lasers, a green-emitting diode laser would enable high quality full color projection displays for a very large commercial market.
While green-emitting diode lasers based on the II-VI material system have been demonstrated, their operating lifetime was so short that they could not achieve commercial viability, and development of such II-VI lasers has largely been abandoned [5]. Argon ion gas lasers operating at 488 nm and 514 nm are widely used for several of the applications listed above, but are far too large and inefficient for use in mass markets.
Frequency-doubled solid state lasers emitting at 532 nm, pumped by flashlamps or near infrared diode lasers, are also in widespread use for scientific and material processing applications. The smallest versions of these frequency-doubled diode pumped solid state lasers do manifest many of the advantages of diode lasers: they are small, lightweight, and are capable of high frequency modulation. Yet compared to diode lasers directly emitting the desired wavelength, the frequency-doubled Diode Pumped Solid State (DPSS) lasers are inefficient, low-powered, and costly to manufacture. For example, state-of-the-art miniature frequency-doubled DPSS lasers [6] attain 61% coupling efficiency between the pump laser and the doubling crystal, and 58% efficiency in the doubling crystal. Combined with the 47% electrical-to-optical efficiency of the pump laser, the overall wall plug efficiency of the doubled DPSS module is only 17%, one third that of the original pump laser. In addition, the lenses, and doubling crystal, and the careful alignment needed during manufacture add significant cost to the frequency doubled DPSS laser, costs not incurred by a direct emitting diode laser.
Diode lasers based on the (Al,Ga,In)As material system have been commercialized, with direct emission wavelengths from approximately 1000 nm down to 630 nm. Light emitting diodes based on (Al,Ga,In)P have been commercialized at wavelengths down to the orange range, 570-590 nm, but laser operation has not been achieved in that material system at that wavelength range. Diode lasers based on the (Al,Ga,In)N material system have been commercialized with direct emitting wavelengths from 370 nm up to 470 nm, and record operation up to 510 nm [7], and while green LEDs have been made in (Al,Ga,In)N, laser operation in the green spectral range remains elusive. To obtain green emission, the indium mole fraction of the active region must be increased to approximately 25%, compared to 10-15% used in violet emitting lasers and 20% in true blue lasers. It is widely believed that the limitation in the (Al,Ga,In)N system is that the active region material quality degrades during subsequent crystal growth of the upper waveguide and electrical contact layers used in conventional diode lasers, and that this degradation worsens as the indium content, temperature, and growth time increase.
One solution is to develop low-temperature crystal growth techniques such as plasma-assisted molecular beam epitaxy (PAMBE) or ammonia molecular beam epitaxy (NH3-MBE), which can use growth temperatures significantly lower than those used in the more common growth technique, metalorganic chemical vapor deposition (MOCVD). Indeed, violet-emitting (Al,Ga,In)N diode lasers grown entirely by PAMBE have been demonstrated [8]. It is not yet established that molecular beam epitaxy (MBE) growth will be able to produce green-emitting lasers, for the low growth pressure compared to MOCVD does not favor high indium incorporation or high crystal quality.
Other solutions include a number of different techniques as described below:
Kim [9] has grown single crystal epitaxial zinc oxide (ZnO) layers on (Al,Ga,In)N LEDs using a low temperature hydrothermal method, for the purpose of improved light extraction from LEDs. ZnO lateral epitaxial overgrowth (LEO) onto (Al,Ga,In)N Light Emitting Diodes (LEDs) was also described [10]. The ZnO formed a transparent electrical contact, with no waveguiding function, and use with diode lasers was not discussed.
Sasaoka [1] has described a method to form (Al,Ga,In)N ridge waveguides by high temperature LEO of (Al,Ga,In)N layers through an opening in an AlN mask, for the purpose of improving the performance of lasers emitting violet or blue light. The regrown waveguides performed both electrical and optical functions, but were grown at temperatures that would degrade (Al,Ga,In)N active regions intended for green emission.
Fang [2] has described a technique to bond semiconductor active regions formed from (Al,Ga,In)As and (Al,Ga,In)(As,P), grown on GaAs or InP substrates respectively, to ridge waveguides formed in silicon-on-insulator substrates, specifically for integration of III-V optoelectronic components with silicon electronics. The silicon ridge waveguides were electrically passive, unlike the conducting waveguides proposed in this application. They have demonstrated working electrically-pumped lasers, photodetectors, and amplifiers operating at near infrared wavelengths. They did not claim any applicability to other material systems.
Sink [3] has described a technique to bond (Al,Ga,In)N laser structures to cubic substrates such as GaAs or InP, with subsequent removal of the growth substrate, specifically to facilitate the cleaving of high quality laser facets. The cleavage substrate performed no optical role.
Murai [4] has described a technique to bond bulk ZnO to (Al,Ga,In)N LED structures for the purpose of enhanced light extraction from LEDs. The bonded ZnO performed both an electrical and optical role (See also, U.S. patent application Ser. No. 11/454,691, filed on Jun. 16, 2006, by Akihiko Murai et. al, entitled “(Al,Ga,In)N AND ZnO DIRECT WAFER BONDING STRUCTURE FOR OPTOELECTRONIC APPLICATIONS AND ITS FABRICATION METHOD,” which application is incorporated by reference herein.)
Margalith [15] has described transparent electrical contacts to (Al,Ga,In)N, formed by sputter deposition of indium tin oxide (ITO) or titanium nitride, for the purpose of enhanced light extraction from LEDs and for low loss electrical contacts to vertical cavity surface emitting lasers. Surface emitting laser operation was not achieved, and no waveguiding function or other use with in-plane lasers was proposed.
The key invention described here is to use unconventional designs and fabrication methods to eliminate the prolonged growth at high temperature after the laser active region is formed, while retaining the benefits of MOCVD growth of the active region